1. Field of the Invention
The present invention relates to a piston for a cylinder injection engine which is of a spark ignition type and in which fuel is injected directly into a combustion chamber.
2. Description of the Related Art
Internal combustion engines of a type in which fuel is injected within a cylinder are generally classified into a so-called cylinder-injection internal combustion engine and a so-called direct-injection internal combustion engine, and diesel engines have been widely known. However, in recent years, cylinder injection engines of the spark ignition type have been proposed as well (since spark ignition engines are generally gasoline engines, spark ignition engines will be referred to as "gasoline engines").
FIG. 6 shows a cross section of a combustion chamber and its vicinity within a typical cylinder injection engine.
In the cylinder injection engine shown in FIG. 6, a cylinder head 13 is securely fixed onto a cylinder block 11 via a cylinder head gasket 12 and through use of unillustrated bolts. A piston 15 is placed in a cylinder 14 within the cylinder block 11 such that the piston 15 is movable in the vertical direction. The cylinder block 11, the cylinder head 13, and the piston 15 define a combustion chamber 16, to which an intake port 17 and an exhaust port 18 are connected. The lower end portion of an intake valve 19 is situated in the intake port 17, while the lower end portion of an exhaust valve 20 is situated in the exhaust port 18. Unillustrated intake and exhaust cams are in engagement with the respective upper end portions of the intake valve 19 and the exhaust valve 20. When the intake and exhaust cams are driven, the intake valve 19 and the exhaust valve 20 are operated to establish and break fluid communication between the combustion chamber 16 and the ports 17 and 18.
Also, a fuel injection valve 21 for injecting fuel into the combustion chamber 16 is attached to the cylinder head 13 in a manner such that the lower tip of the fuel injection valve 21 faces the combustion chamber 16. The fuel injection valve 21 receives pressurized fuel from an unillustrated fuel pump and injects the pressurized fuel into the combustion chamber 16. Further, a spark plug 22 is attached to the cylinder head 13 such that the tip of the spark plug 22 faces the combustion chamber 16. The spark plug 22 ignites the fuel mist injected from the fuel injection valve 21 into the combustion chamber 16.
Accordingly, when an unillustrated crankshaft rotates, the piston 15 reciprocates within the cylinder 14 via a connecting rod. Meanwhile, the rotational drive force of the crankshaft is transmitted to the unillustrated camshaft so that the camshaft rotates synchronously with the rotation of the crankshaft. Thus, the intake valve 19 and the exhaust valve 20 are opened and closed by the intake cam and the exhaust cam. At this time, air is taken from the intake port 17 into the combustion chamber 16, and fuel is injected from the fuel injection valve 21 into the combustion chamber 16. Thus, a swirl flow of fuel mist is generated and ignited by the spark plug 22, so that a cycle comprising an intake stroke, a compression stroke, a power or expansion stroke, and an exhaust stroke is repeated.
In such a cylinder injection engine, a cavity (depression) is formed in the top face of the piston 15 in order to allow the fuel-air mixture to swirl within the combustion chamber 16 to burn efficiently when being ignited by the spark plug 22. A piston having a cavity in the top face is disclosed in, for example, Japanese Utility Model Application Laid-Open (kokai) No. 5-21132. However, in the piston disclosed in this publication, the depth of the cavity is increased on the exhaust valve side, and thus fuel mist is not caused to swirl, and therefore is not led to the spark plug. Therefore, the shape of the cavity is not suitable for a cylinder injection engine. In order to solve the above problem, the applicant of the present invention filed a patent application (Japanese Patent Application No. 7-233129) for an invention related to the shape of a piston having a cavity.
Each of FIGS. 7A, 7B, and 7C schematically shows a piston having a cavity for which the present applicant filed a patent application.
In the piston 101 shown in FIG. 7A, on the side where the intake valve is disposed, an intake-valve-side slant top face 102 is formed such that the top face 102 inclines upward toward the center portion of the piston, and on the side where the exhaust valve is disposed, an exhaust-valve-side slant top face 103 is formed such that the top face 103 inclines upward toward the center portion of the piston. Thus, the top face of the piston 101 has a pent-roof-like shape. A cavity 104 is formed in the intake-valve-side slant top face 102 such that the bottom surface of the cavity 104 is part of a virtual spherical surface centered at a point above the unillustrated intake valve. Accordingly, the cavity 104 of the piston 101 reverses the flow of air that enters the combustion chamber toward the top face of the piston 101 in order to cause the air to flow toward the lower surface of the cylinder head. Thus, generation of tumble flow (vertical swirl) is assisted. The fuel injected into the combustion chamber is guided to the cavity 104 and is then conveyed by means of the tumble flow from the periphery of the combustion chamber to the center portion of the combustion chamber.
In the piston 201 shown in FIG. 7B, on the side where the intake valve is disposed, an intake-valve-side slant top face 202 is formed, and on the side where the exhaust valve is disposed, an exhaust-valve-side slant top face 203 is formed. Thus, the top face of the piston 201 has a pent-roof-like shape. A cavity 204 is formed in the intake-valve-side slant top face 202. The cavity 204 is composed of a flow-entrance portion 205, a rising portion 206, and a connection portion 207. The flow of air that enters the combustion chamber toward the top face of the piston 201 is reversed in order to cause the air to flow toward the lower surface of the cylinder head. Thus, generation of tumble flow (vertical swirl) is assisted. The fuel injected into the combustion chamber is guided to the cavity 204 and is then conveyed by means of the tumble flow from the periphery of the combustion chamber to the center portion of the combustion chamber.
In the piston 301 shown in FIG. 7C, an intake-valve-side slant top face 302 and an exhaust-valve-side slant top face 203 are formed as in the piston 101 of FIG. 7A, so that the top face of the piston 301 has a pent-roof-like shape. A cavity 304 is formed in the intake-valve-side slant top face 302. The cavity 304 is defined by two virtual spherical surfaces 305 and 306 centered at positions above the intake valve and a connection surface 307 connecting the virtual spherical surfaces 305 and 306. Accordingly, the cavity 304 of the piston 301 reverses the flow of air that enters the combustion chamber toward the top face of the piston 301 in order to cause the air to flow toward the lower surface of the cylinder head. Thus, generation of tumble flow (vertical swirl) is assisted. The fuel injected into the combustion chamber is guided to the cavity 304 and is then conveyed by means of the tumble flow from the periphery of the combustion chamber to the center portion of the combustion chamber.
Further, U.S. Pat. No. 5,127,379 discloses the structure of a cylinder injection engine in which a spark plug is disposed at the center portion of the combustion chamber; an injection valve is disposed at the periphery of the combustion chamber; a depression is formed on the top face of the piston such that the depression extends from a point below the spark plug to a point below the injection valve; and a fuel guide groove narrower than the depression is formed on the inner wall surface of the depression at a location below the spark plug, such that the fuel guide groove extends from the side portion of the depression toward the spark plug.
In the cylinder injection engine described in the above-described Japanese Patent Application No. 7-233129, the cylinder diameter or the diameter of the piston varies depending on the displacement of the engine and the number of cylinders. If a piston having a certain diameter is designed to have the shape of the above-described piston 101, 201, or 301, the size of the cavity 104, 204, or 304 is increased or decreased in proportion to the diameter of the piston. However, when the size of the cavity 104, 204, or 304 is increased with the diameter of the piston, the curvature of the cavity 104, 204, or 304 decreases, and the volume of the cavity 104, 204, or 304 increases. As a result, the air-fuel mixture becomes leaner in the vicinity of the spark plug, with a fear that stratified combustion does not occur.
The cylinder injection engine disclosed in U.S. Pat. No. 5,127,379 is also configured in such a way that a vertical swirl flow is generated in the air that flows from the intake portion into the combustion chamber. The generation of the vertical swirl flow that starts in an intake stroke ends with the completion of the intake stroke, and in a subsequent compression stroke, the swirl flow is preserved by means of only inertia of the swirl flow. As described in this U.S. patent, since the fuel guide groove formed in the top face of this piston is narrower than the depression, an inflection point (corner portion) where the curvature changes drastically exists at the connection portion between the depression and the fuel guide groove as viewed from above. Therefore, the vertical swirl flow generated during the intake stroke causes a turbulence at the inflection point, so that the vertical swirl flow cannot be preserved up to the latter half of the compression stroke. Consequently, fuel cannot be guided by means of vertical swirl flow; the operation range in which reliable ignition and stable combustion are possible is limited; and a stratified combustion cannot be realized.